1. Field of the Invention
This invention relates in general to automobile lifting and towing equipment, and in particular to a system for securing the wheels of a vehicle to be towed to the cross-arm of a wheel-lift tow truck.
2. Description of Related Art
Until recently, tow trucks traditionally used a sling system to attach vehicles, lift them, and tow. The sling is suspended from a boom at the rear of the tow truck and stands off from the back plate of the tow truck body by means of arms which are pivotally connected to the backplate. The vehicle to be towed is typically attached to the sling by chains which are hooked onto the axle or frame of the vehicle and which are tensioned as the tow truck boom raises the sling and vehicle. The chains force the vehicle against the flexible members of the sling (typically a pair of rubber or nylon straps) and keep the vehicle suspended above the ground as it is towed.
Recent automotive design changes have included lowering of the automobile chassis and incorporation of low air dams and bumpers which are less rigid than the steel bumpers of earlier model years. These changes make towing newer vehicles with a sling system difficult to do without damaging the bumpers and air dams. As a result, new systems for attaching and towing vehicles which are based on securing the wheels of the towed vehicle to the tow truck have been developed by tow truck manufacturers. These are called wheel-lift towing systems.
Wheel-lift tow truck designs typically incorporate a telescoping or folding tube or "stinger", at the end of which a cross-arm is pivotally connected. When being positioned for towing a vehicle, the cross-arm is placed against the tread of the tires of the vehicle. The cross-arm may or may not have sliding extensions to make the cross-arm length adjustable to different track dimensions of towed vehicles. Wheel retainers, which are separate from the stinger/cross-arm system or which are pivotally connected to the cross-arm or cross-arm extensions, are secured to the cross-arm or cross-arm extensions manually to entrap the wheel so that the vehicle may be lifted by raising the stinger.
The typical wheel-lift system can be used as the sole system for lifting and towing on a tow truck, or it can be used in combination with a boom and sling system on a tow truck. The versatility of such combination tow trucks is useful to tow truck operators who need the ability for recovering vehicles which have gone off the road. The operator typically uses a winch system built into the boom assembly on combination units to pull the vehicle to be recovered up to a location near the tow truck so that it can be attached, lifted, and towed. Furthermore, a sling system is a preferred method for towing some types of vehicles (usually the heavier ones) when possible, since its use is less detrimental than the use of a wheel-lift system to the weight distribution of the tow truck when loaded with a given vehicle.
Wheel-lift designs have been further refined to incorporate the ability for the wheel-lift to be operated remotely. These refinements reduce the time and effort required for securing and lifting the towed vehicle. In one example disclosed in U.S. Pat. No. 4,473,334, issued Sep. 25, 1984 to Brown, claw-shaped wheel retainers are pivotally connected to the cross-arm, and the wheels of the towed vehicle are entrapped by extending the stinger and cross-arm toward the vehicle and allowing the wheels of the vehicle to cause the claws to pivot into the correct position for lifting and towing. This system has many disadvantages. It cannot conveniently be used in snowy climates, since the claws may not pivot freely when they come into contact with ice or snow near the wheels of the vehicle. The stinger must be folded rather than telescopically retracted into the tow truck body for storage, since the claws when in the stored position protrude too far to the rear of the tow truck. This makes it impossible for the claw wheel-lift system to be incorporated with a sling system on the same tow truck. It also requires the stinger system to be located well behind the axle of the tow truck, since the truck frame must be adequately strengthened with a cross-member between the axle and the folding stinger.
In another example disclosed in U.S. Pat. No. 4,564,207, issued Jan. 14, 1986, to Russ et al., L-shaped wheel retainers are pivotally connected to the cross-arm and are stored with the legs of the "L" together and extending back from the cross-arm in the same plane as the stinger and cross-arm. To load a vehicle, the cross-arm is placed against the tread of the tires to be lifted, with the legs of the "L" extending underneath the vehicle between the tires. The Ls are then pivoted into position behind the tires by means of hydraulic cylinders acting directly on the plate at the base of the Ls. This system suffers some of the same drawbacks as the prior example cited: it is difficult to use with a recovery boom and sling on the same truck, and it must be used with a folding stinger, since the Ls protrude too far behind the tow truck to keep the stinger in the horizontal or near-horizontal position while driving the tow truck without a load.
In another example disclosed in U.S. Pat. No. 4,678,392, issued Jul. 7, 1987, to Capers et al., the system solves the problem of the storage position by pivotally connecting L-shaped retainers in such a way that the L collapses against the cross-arm and the leg of the L extends forward from the cross-arm when in the stored position. However, the Ls must be pivoted manually, and the system, therefore, cannot be operated completely from a remote position.